One such attempt is disclosed in U.S. Pat. No. 4,448,679, on which the preamble of the independent claims of this application is based. This document relates to a centrifugal FFF apparatus in which a liquid to be fractionated is led into a channel present between an inner ring and an outer bowl-shaped structure of a rotor which can be rotated by a motor. During rotation, the liquid to be fractionated is fractionated by the action of the centrifugal force as well as the flow of the liquid through the channel.
A recurring problem with the prior art has been to reduce the leakage of sample fluid introduced into the field-flow fractionation apparatus where the leaks frequently occur at the inlet of the sample fluid into the apparatus: as the rotor rotates whereas the fluid supply is generally stationary, there is a need for a tight seal of the connection between the rotating and the stationary parts of the FFF apparatus. This is particularly challenging due to the need to prevent a leak occurring at an interface between a stationary and a moving part, which is more difficult than just sealing off two stationary parts.